ArtifactDeploymentService now resolves each site's secret: references (via
the new SiteSecretsCollector, wrapping ISecretStore) into DeployArtifactsCommand.Secrets,
never plaintext. A referenced secret that is absent or tombstoned fails
that site's deploy before anything is sent, with a named error, while the
rest of the batch still deploys; ComputeCurrentArtifactHashesAsync instead
folds a MissingSecrets marker into the hash so such a site reads Out of
date. Central logs what it ships (name@revision) and errors on what's
missing, never material.
Also updates the two other direct ArtifactDeploymentService construction
sites (PendingUpdatesEvaluatorTests, CentralUI SitesPageTests) for the new
constructor parameter.
Joseph Doherty